An online UPS with improved input-output characteristics

Standard uninterruptible power supply (UPS) systems are connected in series between the AC mains and the critical load. A phase-controlled rectifier feeds a battery-supported DC bus and an inverter supplies the load. These systems require two conversion stages. Input power factor is poor and large harmonic currents are injected into the AC mains. A topology is proposed which overcomes these drawbacks by placing the inverter-battery subsystem in shunt between the AC mains and the load. The battery is charged by the inverter DC bus. In normal operation, the AC mains feed the load directly, and the inverter supplies only a small amount of reactive power required to maintain the power factor very close to unity and to regulate the load voltage. The system has the following advantages: a near unity power factor, reduced line current harmonics and higher efficiency. The operation of the proposed topology and design considerations are analyzed, and operating characteristics are verified